feat: add explain support for `SELECT` queries

[BobdenOs]

It is best practice when using HANA to use execution plan visualization files to identify the root cause of slow performing queries. This change targets to add native support for generating and downloading the plan visualization files directly from the HANA service.

As all public facing documentation only instructs on how to use execution plan visualizations with HANA Studio (deprecated for HANA Cloud) and Hana Runtime Tools (Only available in development environments). It is difficult to understand how to get access to the execution plan visualization files and their contents. Therefor providing a direct API in the HANA Service will allow us to direct our users to a clear path to get a better insight into their application performance.

The ability to open the generated files is available in the Visual Studio Code plugin SAP HANA SQL Analyzer.